The fluidized catalytic cracking of hydrocarbons is the main stay process for the production of gasoline and light hydrocarbon products from heavy hydrocarbon charge stocks such as vacuum gas oils or residual feeds. Large hydrocarbon molecules, associated with the heavy hydrocarbon feed, are cracked to break the large hydrocarbon chains thereby producing lighter hydrocarbons. These lighter hydrocarbons are recovered as product and can be used directly or further processed to raise the octane barrel yield relative to the heavy hydrocarbon feed. The basic equipment or apparatus for the fluidized catalytic cracking of hydrocarbons has been in existence since the early 1940's and, along with its method of operation, is well known to those skilled in the an of hydrocarbon processing.
The cracked products from an FCC reaction section are delivered directly to product separation facilities associated with the FCC unit. These separation facilities include a primary separator, often referred to as a main column, and a compression section containing numerous separators and contactors for further separating overhead vapors from the main column. The compression section is commonly referred to as the gas concentration section. A key component of the compression is referred to as the wet gas compressor, which is a main source of energy for the gas concentration section.
Salt deposition in the primary separators of some FCC processes has created problems where solid salts form and accumulate on the surfaces of trays and cause plugging problems. The source of salt formation is primarily nitrogen compounds that enter with the FCC unit feed. Conditions in the reactor convert about 20-40 percent of the nitrogen compounds in the feed to ammonia which in the presence of sulfides or chlorides leads to the formation of water soluble salts such as ammonium chloride and ammonium sulfide. Salting in the trays principally occurs when the concentrations of dissociated salts exceed their saturation limits in the vapor of the primary separator. Precipitation of salts poses the most problems for the upper section of the primary separator where operating temperatures are the coldest and where vapor flow rates are generally the lowest.
The capacity of the separator to carry the salt as vapor is largely determined by the temperature of the vapor itself. The salting occurs more readily as the temperature of trays in the column drop. Lower tray temperature increasingly occur as the cut point of the overhead gasoline vapors is reduced to meet product specifications or as the proportion of water, and lighter hydrocarbons increase due to the severity of the cracking occurring in the reactor. Changes in upstream operating conditions such as the amount of heat extracted at the lower section of the column and lower operating pressures can also depress the temperature at the upper trays thereby increasing salting problems. In the FCC process lowering the reactor temperature to maximize distillate production would have a corresponding depression on temperatures in the main column. On the other hand, operating the reactor at very high temperatures to maximize conversion would generate a higher proportion of dry gas which would also tend to reduce the dew point temperature of the column overhead vapor. The condition of the catalyst also affects the generation of light gases. For example, if metal poisoning is severe, a high amount of H.sub.2 will be generated.
Other operational changes, such as the quantity of steam used in upstream or downstream processing, can cause the vapor at the upper end of the column to approach or exceed water saturation limits. As the water concentration at the top of the separator column approaches saturation limits, it begins to condense on the trays and does not leave the system. As the water becomes saturated with these highly water-soluble salts the condensation of the water causes the salts to precipitate out on the tray with resultant plugging and corrosion problems.
The tendency to process increasingly heavier and contaminated (i.e. dirty feeds) raises the frequency and severity of salt precipitation problems. A direct remedy for preventing the precipitation of salts in separators has been upstream treatment of the hydrocarbon feed to the separator to remove salts and salt precursors. The expense and extra processing steps required for such treatments make them economically undesirable. Moreover, higher salt and contaminant levels raise the possibility of salt precipitation problems persisting even after such treatment.
Another known solution is the installation of a hot reflux system which enhances the exit of the salt with the hotter overhead vapors that are generated and withdrawn from the primary separator. Normally the hot reflux system partially cools the overhead vapor from the separator and recovers the condensate resulting from the partial cooling of the vapors in a hot reflux drum at higher temperature than the typical temperature of the net overhead product. The hot reflux drum provides a phase separation for withdrawal of a stream containing a high salt concentration aqueous phase and circulation of a hotter and heavier hydrocarbon stream back to the column which increases the temperature of the overhead section of the column. The hot reflux does not offer a completely satisfactory solution since it requires several additional pieces of equipment and the drum tends to be large since it receives the entire overhead vapor stream. In addition, there are many cases where, despite its cost, the addition of a hot reflux drum may not offer much advantage. For example, where the desired overhead cut is very light or when there is a significant proportion of LPG components in the overhead stream, the dew point of the total overhead material still remains low such that a salt saturated aqueous phase condenses out inside the separator and the salt removal from the reflux material is still insufficient to prevent salt accumulation and deposition on the trays.